1. Prior Art
The invention relates to a circuit arrangement with a flow probe operated in a bridge circuit, a device for determining bridge balance, which controls a flip flop circuit which can be supplied by an oscillator and controls a switching device for the power supply of the bridge circuit, as is known for example from German Offenlegungsschrift 3,608,538. In the case of the known circuit arrangement, the flow probe is designed as a hot-wire flow probe, the output signal of which is proportional to an operating voltage. The diagonal voltage of the bridge circuit, in which the flow probe is arranged, is fed to the input terminals of a comparator, the output terminal of which is connected to an input of the bistable flip flop stage, the output of which controls a transistor, which applies the operating voltage to an integrator circuit to upon closing of a switch. With the known circuit arrangement, it is to be achieved that an otherwise necessary high-precision voltage reference source in a control unit which evaluates the output signal can be dispensed with. The output frequency of the known circuit arrangement lies in the range between 1 and 10 kHz.
A circuit arrangement for the digitization of an analog signal derived from a measuring probe with a voltage-frequency converter, in which an electrical control device for a fuel injection for internal-combustion engines is provided with an air flowmeter, in which a resistor arranged in a bridge circuit has a heating current passed through it, has become known for example from German Patent Specification 2,448,304. The heating current has a direct-current component and a superimposed alternating-current component, which consists of heating-current pulses of constant duration, the pulse repetition frequency varying as a function of the air flow. To a diagonal of the bridge circuit there is connected an operational amplifier, the output voltage of which is converted with a voltage-frequency converter into a frequency proportional to the quantity of air taken in, which on the one hand controls the alternating current fed to the bridge circuit and on the other hand serves indirectly as controlled variable of the electronic control device for fuel injection metering. The output frequency of the voltage-frequency converter is subjected to a linearization function, which can be realized for instance by a squaring circuit. With this circuit arrangement, a further processing of the pulse repetition frequency signal corresponding to the quantity of air taken in is to be performed in a digital way.
In German Offenlegungsschrift 3 ,037,340, a driver for a hot-wire air flowmeter is described, which has a hot wire for the radiation of heat into an air stream and a resistor for temperature compensation. The voltage, drop at the hot wire is compared with the voltage drop dropping at the compensating resistor. This voltage drop drop difference is integrated and the integrated voltage signal is subjected to a second comparison with the output voltage of a sawtooth generator. On the basis of the result of this second comparison, the pulse duty factor of the constant current through the hot wire is controlled such that the difference between the temperature of the hot wire and the temperature of the compensating resistor is constant. In the case of this known circuit, two constant-current sources are necessary for the feeding of constant currents into the hot wire and into the compensating resistor, respectively.
2. Advantages of the invention
The circuit arrangement according to the invention, with a flow probe operated in bridge circuit, a device for determining bridge balance, which controls a triggering circuit which can be supplied by an oscillator and controls a switching device for the power supply of the bridge circuit, in which the oscillator has an operating frequency which is at least approximately inversely proportional to the square of the operating voltage of the power supply has, in particular, the advantage that now only semiconductors are necessary for switching control, the power of which may be lower than in the case of the prior art. The circuit arrangement according to the invention is therefore less susceptible to faults, is of simpler design and is therefore less expensive. The output signal of the circuit arrangement according to the invention is a pulse duty factor, and this allows a direct evaluation of the output signal in digital control units. The output signal is in dependant of the supply voltage and is therefore also not influenced by any fluctuations of the supply voltage (operating voltage).